Changes in Cell Wall-bound Phenolic Compounds and Lignin in Roots of Date Palm Cultivars Differing in Susceptibility to Fusarium oxysporum f. sp. albedinis

The roots of date palm contain four cell wall‐bound phenolic acids identified as p‐hydroxybenzoic, p‐coumaric, ferulic and sinapic acids. The ferulic acid represents the major phenolic compound since it constitutes 48.2–55.8% of cell wall‐bound phenolic acids. All these phenolic acids were present in the resistant cultivar (BSTN) and the susceptible cultivar (JHL). However, the pre‐infection contents of p‐coumaric, ferulic and sinapic acids were greater in the resistant cultivar than in the susceptible one. For the contents of p‐hydroxybenzoic acid, there was no significant difference between the resistant cultivar and the susceptible cultivar. Similarly, the pre‐infection contents of lignin were approximately equal for both cultivars. Inoculation of the date palm roots by Fusarium oxysporum f. sp. albedinis induced important modifications to the contents of the cell wall‐bound phenolic compounds and lignin, which made it possible to distinguish between resistant and susceptible cultivars. The post‐infection contents of cell wall‐bound phenolic compounds underwent a rapid and intense increase with a maximum accumulation on the tenth day for p‐hydroxybenzoic acid (1.54 μmol/g), p‐coumaric acid (2.77 μmol/g) and ferulic acid (2.64 μmol/g) and on the fifteenth day for sinapic acid (1.85 μmol/g). The maximum contents accumulated in the resistant cultivar were greater than those in the susceptible cultivar, namely, 11 times for p‐hydroxybenzoic acid, 2.6 times for p‐coumaric acid, 1.8 times for ferulic acid and 12.3 times for sinapic acid. In the susceptible cultivar, p‐coumaric acid and ferulic acid contents also increased after inoculation although they did not reach the pre‐infection contents of the resistant cultivar. The contents of p‐hydroxybenzoic acid in the susceptible cultivar roots did not present post‐infection modification and those of sinapic acid decreased instead. The lignin contents increased in both cultivars with a maximum accumulation on the fifteenth day. However, the maximum contents accumulated in the resistant cultivar roots were 1.5 times greater than those of the susceptible cultivar. These results showed clear differences between the resistant BSTN and the susceptible JHL cultivars. The implication of cell wall‐bound phenolic compounds and lignin in the resistance of date palm to F. oxysporum f. sp. albedinis appears to be dependent on the speed and intensity of their accumulation with greater contents in the first stage of infection.